The build-up of static electric charge in moving vehicles can have a potentially adverse effect on electronic circuitry and radio reception, as well as being a safety hazard should spark discharge occur during re-fuelling. Personal discomfort also occurs if charge is earthed through a driver or passenger.
Charge is normally conducted to earth through the tires, which requires that the tires themselves have an acceptable level of conductivity--commonly described as anti-static. Such levels of conductivity are normally guaranteed by the use of tire compounds containing significant proportions of carbon black as reinforcing filler. Carbon black reduces the resistivity of the intrinsically non-conductive rubber and so provides a leakage path for the static charge from the vehicle to earth.
Anti-static rubber compounds generally require a volume resistivity below 10.sup.8 ohm-cm, which is usually achievable with approx 50 phr or more carbon black in the compound. Such loadings are typical of carbon black-reinforced tires, which are accordingly anti-static.
If non-black fillers (e.g. silica) are used, or if the carbon black loading is significantly less than 50 phr, anti-static properties are often not achieved. For example, with silica filler in place of carbon black, the volume resistivity is typically 10.sup.13 ohm-cm. Should such a compound be used in the tread of a tire, for example, to achieve a lower rolling resistance, the vehicle will effectively be insulated from the road and static charge will not be able to leak to earth.
It is known that electrical conductivity may be improved by the addition of polar ingredients but the effect is generally of insufficient magnitude. Likewise special grades of superconductive carbon blacks may be added but these need to be present in moderately high loadings, e.g. 10-30 phr, to be significantly effective, and since their other properties are not suited to tire applications, road performance may deteriorate.
According to one aspect of the present invention a tire tread comprises an elongated tread strip of tire tread compound of high resistivity having a transverse width (TR) and, within the transverse width (TR) a longitudinally extending conducting strip of low resistivity tire compound with a volume resistivity less than 10.sup.8 ohm-cm, the said conducting strip extending from the top to the bottom surface of the tread strip.
By high resistivity is meant tire tread compounds having a volume resistivity greater than 10.sup.89 ohm-cm. Such compounds include, for example, tire tread compounds having a silica filler in place of carbon black of which the volume of resistivity is typically 10.sup.13 ohm-cm.
Preferably the conducting strip is positioned at the center of the tire tread strip.
Two or more longitudinally extending conducting strips may be provided and when two strips are used they may conveniently be the tread strip wing component which in the finished tire form the tire tread shoulders. This construction is particularly convenient because the tire tread wings are usually extruded of a different material to the main part of the tire tread strip and thus no additional extruded component is necessary.
The width of the conducting strip is preferably in the range of 5-25% of the width of the tire tread TR.
The compound of the conducting strip more preferably has a volume resistivity of less than 10.sup.6 ohm-cm.
Another aspect of the invent ion provides a tire comprising a tire tread strip according to any of the above claimed features. Preferably the width of the conducting strip is greater than the width of the widest longitudinally extending tread groove to ensure that the conducting strip ma y contact the road when the tire is completed and used on a vehicle.